Kirsten Rat Sarcoma Oncogene Homolog (KRAS) is a small GTPase that integrates signals from outside the cell to proliferation and survival signals within the cell. This occurs through growth factor-mediated activation of Guanine Exchange Factors (GEFs), which remove GDP from Ras and allow the entry of GTP, which resides in high concentrations in the cytoplasm. Upon binding of the GTP nucleotide, two disordered switch regions (switch I and switch II) interact with the gamma phosphate of the nucleotide allowing Ras to interact with effector enzymes via a Ras Binding Domain (RBD), which start signalling cascades that alter gene expression. Binding of a GTPase activating protein (GAP) accelerates the intrinsic conversion of GTP to GDP and renders the protein in an inactive state thus terminating the signal (Rajalingam, K., R. Schreck, U. R. Rapp and S. Albert (2007). “Ras oncogenes and their downstream targets.” Biochim Biophys Acta 1773(8): 1177-1195.)
Ras is mutated in up to 20% of human tumors at the codon 12, 13, and 61 positions, which serve to promote the GTP bound form of the protein. These include colon, pancreas and lung tumors, the latter of which show KRAS mutation in up to 25-30% of all tumors with 40% of these harboring a G12C mutation thought to be promoted by carcinogens in cigarette smoke. KRAS with G12C mutations activate the Mapk pathway and promote Non-Small Cell Lung Cancer (NSCLC) growth and survival. (Prior, I. A., P. D. Lewis and C. Mattos (2012). “A comprehensive survey of Ras mutations in cancer.” Cancer Res 72(10): 2457-2467.)
Since the discovery of KRAS mutations in human tumors and that inhibiting signalling by these proteins caused inhibition of the cancer phenotype there has been a strong desire by both academic groups and industry to find Ras inhibitors (Feramisco, J. R., R. Clark, G. Wong, N. Arnheim, R. Milley and F. McCormick (1985). “Transient reversion of ras oncogene-induced cell transformation by antibodies specific for amino acid 12 of ras protein.” Nature 314(6012): 639-642.) and (McCormick, F. (2015). “KRAS as a Therapeutic Target.” Clin Cancer Res 21(8): 1797-1801. Specific Inhibitors of the KRAS effector BRaf alone and combined with other inhibitors the Mapk pathway have shown dramatic responses in melanoma where this BRaf is frequently activated via mutation (Flaherty, K. T., I. Puzanov, K. B. Kim, A. Ribas, G. A. McArthur, J. A. Sosman, P. J. O'Dwyer, R. J. Lee, J. F. Grippo, K. Nolop and P. B. Chapman (2010). “Inhibition of mutated, activated BRAF in metastatic melanoma.” N Engl J Med 363(9): 809-819.) In contrast, general Mapk inhibitors have not shown dramatic responses in cancers with mutant KRAS potentially because of the lack of an appropriate therapeutic index over normal tissues or compensatory signalling by other Ras pathways (Turk Turke, A. B., Y. Song, C. Costa, R. Cook, C. L. Arteaga, J. M. Asara and J. A. Engelman (2012). “MEK inhibition leads to PI3K/AKT activation by relieving a negative feedback on ERBB receptors.” Cancer Res 72(13): 3228-3237.e, Song et al. 2012) and (Janne, P. A., M. M. van den Heuvel, F. Barlesi, M. Cobo, J. Mazieres, L. Crino, S. Orlov, F. Blackhall, J. Wolf, P. Garrido, A. Poltoratskiy, G. Mariani, D. Ghiorghiu, E. Kilgour, P. Smith, A. Kohlmann, D. J. Carlile, D. Lawrence, K. Bowen and J. Vansteenkiste (2017). “Selumetinib Plus Docetaxel Compared With Docetaxel Alone and Progression-Free Survival in Patients With KRAS-Mutant Advanced Non-Small Cell Lung Cancer: The SELECT-1 Randomized Clinical Trial.” Jama 317(18): 1844-1853.)
Compounds that selectively bind mutant KRas are highly desirable as they would spare impact on normal tissues and for adequate inhibition of Ras signalling within the tumor to elicit antitumor activity. Recently KRAS G12C has been shown to retain cycling both biochemically and in cancer cells, creating an opportunity to disrupt activation (Hunter, J. C., A. Manandhar, M. A. Carrasco, D. Gurbani, S. Gondi and K. D. Westover (2015). “Biochemical and Structural Analysis of Common Cancer-Associated KRAS Mutations.” Mol Cancer Res 13(9): 1325-1335.) Compounds that utilize the cysteine substitution in G12C for binding and prevent the GDP to GTP exchange were described (Ostrem, J. M., U. Peters, M. L. Sos, J. A. Wells and K. M. Shokat (2013). “K-Ras(G12C) inhibitors allosterically control GTP affinity and effector interactions.” Nature 503(7477): 548-551.) This makes utilizing G12C binding compounds that lock KRAS in the inactive state an attractive opportunity for cancer treatment.